1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, a battery unit having variable margins of protruding portions of a separator interposed between opposing electrode plates, the protruding portions sticking out on either side of each electrode plate, and a lithium secondary battery employing the battery unit.
2. Description of the Related Art
A lithium secondary battery can be classified into a liquid electrolyte battery and a polymer electrolyte battery according to the kind of electrolytic solution used. In general, a battery using a liquid electrolyte is referred as a lithium ion battery, and a battery using a polymer electrolyte is referred to as a lithium polymer battery.
Such a lithium secondary battery can be manufactured in various shapes, typically cylindrical and rectangular shapes mainly used in lithium ion batteries. Lithium polymer batteries, which have recently gained much attention, are manufactured by flexible materials, having relatively high freedom in shape. Also, since the lithium polymer batteries are highly safe and lightweight, they are advantageously used for attaining slim, light portable electronic devices.
A battery unit of a conventional lithium secondary battery includes a plurality of electrode plates having different polarities, and a separator interposed between the electrode plates. The battery unit is wound in a jelly-roll type in which a positive electrode plate, a separator and a negative electrode plate are sequentially disposed.
The electrode plates includes an electrode current collector and an electrode active material layer coated on the surface of the electrode current collector. The electrode current collector includes an electrode uncoated area where the electrode active material layer is not coated. An electrode lead is attached to the electrode uncoated area. A protective tape is wrapped at an end of the electrode lead.
The separator is interposed between the plurality of electrode plates having different polarities in order to insulate the electrode plates from each other, and has a width greater than that of each electrode plate. The entire width W1 of the separator equals the sum of the width W2 of the electrode plate and the widths W3 and W4 of protruding portions of the separator which stick out on either side of the electrode plate.
As described above, since the width of the separator is greater than that of the electrode plate by the sum of widths of the protruding portions of the separator which stick out on either side of the electrode plate, it can prevent the electrode plates having different polarities from contacting each other. Accordingly, electrical short-circuit between the electrode plates can be prevented. Here, the widths W3 and W4 of the protruding portions of the separator are substantially the same with each other.
The conventional battery unit having the above-described configuration can be completed by disposing the separator between the plurality of electrode plate having different polarities and then winding the resultant structure in a jelly-roll type.
The battery unit is housed in a pouch type case. The case includes an upper case member, and a lower case member coupled to the upper case member and having a space in which the battery unit can be accommodated. The upper and lower case members are incorporated at one or more sides for simplifying the manufacturing process.
After the battery unit is housed in the case, predetermined heat and pressure are applied to a sealing plane formed at the edge of the case, to complete a battery assembling work.
However, the conventional battery unit has the following problems.
First, the separator made of a polymer, which is weak against heat, may shrink or deform due to heat applied during heat fusion. In particular, thermal deformation is apt to occur at both edges of shorter sides of the case from which the electrode lead is drawn because heat is transferred thereto during heat fusion of the sealing plane.
The reason of the foregoing is because of a small design margin for the width W3 of the protruding portion of the separator which sticks out on one side of the electrode plate, from which the electrode lead is drawn.
In other words, the width W3 of the protruding portion of the separator corresponds to a width at a joint portion of the upper and lower case members, and is the same as the width W4 of the protruding portion of the separator which sticks out on the other side of the electrode plate, where no thermal deformation occurs.
Thus, if the separator is deformed by heat, the electrode plates having different polarities are electrically connected to each other at the shorter side edge of the case , resulting in a short-circuit.
In order to solve the problem, the widths of the protruding portions of the separator may be increased. However, this approach increases the overall volume of the battery unit, making it difficult to attain a high-capacity battery.
Next, in the case where the case has a structure of a polymer layer, a metal foil layer and a polymer layer sequentially stacked, if the metal foil layer as an intermediate layer is exposed, the separator may shrink or deform so that the separator is brought into contact with the exposed portion, thereby increasing the likelihood of corrosion of the metal foil layer due to a difference in oxidation potential.